ABSTRACT: Project 1 Following encounter of a cognate antigen (Ag), naive T cells (Tn) must make a series of fate decisions that determine their differentiation, function and survival. In the context of a viral infection, CD8 T cells initially proliferate vigorously, giving rise to mostly short-lived effector cells (Teff) and a small number of long-lived memory cells. The rules that govern the kinetics of early and late anti-viral Teff responses and the magnitude, functional differentiation, migratory properties and life-span of virus-specific memory cell subsets are still incompletely understood. Traditionally, memory cells have been categorized into two major subsets, central memory cells (Tcm) and effector memory cells (Tem), which differ in their functional response to recall Ag and in their expression of traffic molecules that are needed for recirculation through lymph nodes (LNs). There are few robust markers to specifically identify Tem and to distinguish them from Tn and Tcm. Indeed, there are hints that Tem are not homogenous but composed of specialized subsets with distinct fates and functions, however, the behavior, longevity and lineage relationships of these subsets are unresolved. This Project will test the hypothesis that the chemokine receptor CX3CR1 may be a robust marker to identify three phenotypically and functionally distinct subsets of Ag-experienced CD8+ T cells: CXR3CR1neg Tcm, CX3CR1high Tem and a previously unknown population of CX3CR1low cells that we have named transitional memory cells (Ttm). This hypothesis will be explored in the two specific aims: Aim 1. To establish and characterize CX3CR1 as a marker for Ag experienced anti-viral CD8 T cell subsets. Aim 2. To characterize the differentiation and function of Ttm in anti-viral immunity. If successful, this highly synergistic project will establish CX3CR1 as a powerful marker to identify, discriminate and study Ag-experienced CD8 T cell subsets and introduce Ttm, a novel type of memory T cell distinct from Tcm and Tem. The role of Ttm relative to other memory cell subsets in anti-viral immunity will be clarified.